Various methods of removal of volatile catalyst poisons, for example, oxygen, carbon dioxide, and carbon monoxide, from liquid olefins and saturated liquid hydrocarbons fed to the polyolefin polymerization system are practiced. One method for removing volatile catalyst poisons from a liquid hydrocarbons feed involves distillation in packed or trayed columns using reboilers. These systems are investment intensive, for example, often including a column, reboiler, condenser, surge tank, aftercooler, and feed pump required for each liquid hydrocarbon stream. These systems are also relatively expensive to operate due to the steam and cooling water requirements. Furthermore, some of the vaporized hydrocarbon is typically lost in the vent as the volatile catalyst poisons are vented from the system.
Another method for removing volatile poisons from a liquid hydrocarbon feed uses a packed bed or column with liquid flowing downward through the bed while an inert gas, such as nitrogen, flows upward. As the inert gas flows upward, volatile catalyst poisons transfer from the liquid into the inert gas. The inert gas is then vented from an upper portion of the bed or column (the column vent stream) and is typically sent to a flare. This system typically requires less investment than a reboiler system, but is relatively inefficient because of the amount of hydrocarbons that are lost in the vent stream exiting the column or bed. The vent losses are especially high for relatively high vapor pressure hydrocarbons, such as butene.
Another method for removing volatile poisons from a liquid hydrocarbon feed uses an inert gas, such as nitrogen, sparged up through a vessel containing the liquid hydrocarbon. This method generally has low investment, but is relatively inefficient in removing volatile poisons and results in relatively high hydrocarbon losses.
In some methods, such as packed bed and sparging systems, the column or vessel vent stream can cause problems in the flare system due to a relatively high concentration of inert gas in the stream, which results in low energy content (referred to as BTU value) and problems with efficient burning of the contained hydrocarbons. In some cases, a hydrocarbon, such as methane, must be added to the flare stream to raise the BTU content to allow efficient burning in the flare. In other cases, the column vent stream is sent to a hydrocarbons distillation or cracking unit to recover the contained hydrocarbons. This also presents problems for the distillation or cracking unit due to the high inert content.
Additionally, many polyolefin polymerization systems use more than one liquid hydrocarbon feed. For example, a polyethylene production unit may feed liquid propylene, butene, hexene, octene, or other liquid alkenes as a comonomer. Common comonomers employed in gas phase reactors are 1-butene, 1-hexene, and 4-methyl-1-pentene. In addition, slurry reaction systems may feed saturated aliphatic and aromatic hydrocarbons, such as pentane, hexane, heptane, octane, toluene, xylene, and cyclohexane and mixtures of solvents. Gas-phase reaction systems may feed alkenes as comonomer and an inert hydrocarbon, such as an alkane, or cycloalkane as an induced condensing agent(s) (ICAs) or simply as an agent to raise the molecular weight or specific heat of the gas. The most common types of ICAs are isopentane and n-hexane, but isobutane, or other hydrocarbons (or halogenated hydrocarbons, e.g., HFCs) of similar boiling points may also be used. The use of ICAs is further explained in U.S. Pat. Nos. 5,352,749, 5,405,922, and 5,436,304.
Many current polyolefin reactor systems typically utilize one stripping system for each liquid hydrocarbon fed to the system. For example, a polyethylene production system may have a butene comonomer stripping system, a hexene comonomer stripping system, and an ICA stripping system. This typically requires investing in three independent columns or packed bed systems. Furthermore, each of the three stripping systems consume energy and/or inert gas independently, and generates three independent vent streams, each containing inert gases, which are sent to a flare system or processed by other facilities.
The polymers produced contain residual gaseous or liquid alkenes and alkanes that are removed from the resin in purging systems. There are various techniques for removing volatile hydrocarbons from polymers. See, for example, U.S. Pat. Nos. 5,749,412, 5,376,742, 4,372,758, 4,197,399, 3,594,356, and 3,450,183, in which generally columnar vessels are used as a purger, referred to as a polymer purge bin, or product purge bin. The purging processes usually comprise conveying the solid polymer to a polymer purge bin and contacting the polymer in the purge bin with a countercurrent inert gas purge stream to strip away the volatile hydrocarbons contained in the polymer.
To increase unit efficiency and reduce environmental emissions, a vent recovery system is typically utilized to recover hydrocarbons from the mixed hydrocarbon/inert purge gas stream exiting the purge vessel. Methods of recovering hydrocarbons from the polymerization unit vent stream include: a) compression and condensation with water and/or mechanical refrigeration (for example cooling to −10° C.); and b) separation via pressure swing absorption (PSA) or membranes.
In a compression and condensation system, such as described in, for example, U.S. Pat. No. 5,391,656, a polymer purge bin vent stream, which contains inert gases, such as nitrogen, and various monomers, is treated in a series of steps that include: cooling to condense a portion of the reactor gas stream; separating and recycling the condensed liquids such as hexene, hexane, butene, isopentane, and the like from the remaining non-condensable gases; compressing the non-condensable gases; cooling the compressed stream to promote further condensing, further liquid/gas separation, and further recycle of condensed monomers. The compression and cooling vent recovery system provide recovery of a high percentage of the heavier contained hydrocarbons through the condensation process.
Another recovery method contemplated in the art involves cryogenic vent recovery, wherein condensation of monomer from vent streams containing nitrogen is accomplished by vaporization of liquid nitrogen (either with or without vent compression and to temperatures as low as and below −100° C.). Commercially available cryogenic vent recovery systems used for cryogenic vent recovery typically rely on importing liquid nitrogen from another facility at site, importing liquid nitrogen from an off-site facility, or sending the vent to an off site to recover the condensable hydrocarbons as a refuse stream.
U.S. Pat. No. 6,576,043 describes a process for the separation of a gas mixture comprising nitrogen and at least one hydrocarbon from a polyolefin production plant in which the gas mixture is separated into hydrocarbon and nitrogen streams in an adsorbent bed by a Pressure Swing Adsorption (PSA) process.
U.S. Pat. No. 6,706,857 describes a process for the production of a polyolefin, wherein an olefin monomer is polymerized and a residual monomer is recovered from a gas stream comprising the monomer and nitrogen. This process also uses a PSA process.
U.S. Pat. No. 5,769,927 describes a process for treating a purge vent stream from a polymer manufacturing operation by condensation, flash evaporation, and membrane separation.
U.S. Pat. No. 6,829,906 relates to recovering volatile compounds and inert gases from vessels, such as barges, that need vapor de-pressuring for changing products or human entry for servicing or inspection, liquid filling, or liquid unloading.
U.S. Pat. No. 4,690,702 discloses a method and apparatus for cryogenic fractionation of a gaseous feed employing a contact purifying refrigeration column and refrigerating fluid circuit.
U.S. Pat. No. 5,741,350 discloses a method and apparatus for recovery of hydrocarbons from polyalkene product purge gas, wherein the alkene monomer is condensed and separated at low temperature from the inert gas, and recycled to the polymerization process.
Other background references include U.S. Pat. Nos. 4,188,793, 4,727,723, 5,035,732, 5,266,276, 5,421,167, 5,497,626, 5,626,034, 5,979,177, 6,063,877, 6,560,989, 6,576,805, 6,712,880, and 7,128,827; U.S. Patent Application Publication Nos. 2005/0229634 and 2005/0159122; and EP-549252-A.
Another consideration in polyolefin polymerization systems is the removal of non-condensable gas from the polymerization system. Removal of non-condensable gases may be required due to a slow increase in non-condensable gas concentration, for example ethane, over time as the system operates. Removal of non-condensable gases may be also be required due to transitioning the reactor from producing a product using a higher concentration of non-condensable gases used in the reaction, for example hydrogen, to a product that uses a lower concentration of the non-condensable gas. In either case, reactor gas containing valuable monomers and comonomers are often vented from the polymerization system in order to remove the non-condensable gases. In some cases, considerable volumes of reactor gases must be vented in order to remove the desired amount of non-condensable gases, resulting in significant monomer and comonomer loses.
In view of the considerations discussed above, there exists a need to provide a cost effective method of removing volatile catalyst poisons from multiple liquid hydrocarbon feed streams being fed to polyolefin production systems. Furthermore, there exists a need to reduce the levels of non-condensable gases in the polymerization system without losing valuable monomers and/or comonomers contained in the reactor gas. Still further, there exists a need to reduce the hydrocarbons vented to a flare or recovery systems from liquid hydrocarbon stripping systems. Finally, there exists a need to reduce the amount of low-energy gases, such as nitrogen, being sent to the flare in polyolefin production systems.